Atomic layer deposition (ALD), also known as atomic layer epitaxy, is a process for depositing highly uniform and conformal thin layers of a metal on a surface. The surface is exposed to vapors of the metal precursor and a reducing agent. Such films have a wide variety of applications in semiconductor microelectronics and optical films. The conventional ALD process, which uses a two-step procedure, is described by M. Ritala and M. Leskela in “Atomic Layer Deposition” in Handbook of Thin Film Materials, H. S. Nalwa, Editor, Academic Press, San Diego, 2001, Volume 1, Chapter 2.
In a typical two-step ALD process, there is a self-limiting adsorption of the metal complex to the surface that is controlled by the interaction of the precursor with the substrate in a thermal degradation step. The loss of the ligand is induced thermally, as the metal surface has no functional groups to induce ligand loss chemically. It is desirable that the metal precursor be both stable enough to be transferred into the deposition chamber, and reactive enough to undergo a transformation at the substrate surface.
In a related ALD process, the substrate contains functional groups that react chemically with at least one ligand on the metal-containing precursor. For example, a typical process used to prepare thin, conformal Al2O3 films uses a substrate with hydroxyl groups. The substrate is contacted with Al(CH3)3, which reacts with the surface hydroxyl groups to form an adsorbed Al—O complex and liberated methane. When the surface hydroxyl groups are consumed, the reaction stops. Water is then contacted with the Al—O complex on the surface to generate an aluminum oxide phase and additional hydroxyl groups. The process is then repeated as needed to grow an oxide film of desired thickness.
The deposition rate of the Al(CH3)3 is controlled by the number of surface hydroxyl groups. Once the hydroxyl groups are consumed, no additional Al(CH3)3 can be adsorbed to the surface.
In other known ALD processes for the deposition of metal films on substrates of interest, there may be no reactive group on the substrate surface to initiate the type of self-limiting reaction that is seen in the Al2O3 case. For example, in the deposition of a metal barrier layer on a tantalum nitride substrate, the self-limiting adsorption is achieved through the thermal decomposition of the precursor. The precursor is preferably designed to have the volatility and stability needed for transport to the reaction chamber, but also the reactivity to undergo clean thermal decomposition to allow a metal complex to chemisorb to the substrate surface. Often, these processes produce films contaminated with fragments from the metal ligands degraded during the thermal deposition.
US2002/0081381 describes a process for deposition of metal films (Co, Fe, Ni, Pd, Ru, Rh, Ir, Pt, Au, Ag) by an ALD process. The ligands are chosen from β-diketones, monothio-β-ketones, dithio-β-ketones, aminoketones, and silyl-β-ketones. Reducing agents are hydrogen and silane. Co(acac)2 is disclosed as a precursor for the deposition of cobalt films. Oxygen and other oxidizing agents are disclosed in a process that involves formation and reduction of a metal oxide layer.
WO2004/046417 describes the formation of cobalt films by adsorption of Co(N,N′-diisopropylacetamidinate)2 on a substrate surface and reduction with hydrogen gas. Deposition of cobalt is disclosed, with a deposition temperature of 250-350° C. No deposition was obtained at temperatures less than 250° C.
US 2004/0092096 discloses a method for improving the adhesion between a diffusion barrier film and a metal film, by creating a monolayer of oxygen atoms between the diffusion barrier film and the metal film. Suitable metals include Cu, Al, Ni, Co and Ru. In one embodiment, the monolayer is created by exposing the diffusion barrier film to an oxygen-containing reactant and then depositing the metal film via CVD, ALD, PVD or sputtering.
There is a need for a process for the formation of oxide-free metal-containing films that can be conducted at relatively low temperatures and that can provide high quality, uniform films of high purity.